psc: Introduce ActiveUidsInternal interface

Introduce `ActiveUidsInternal` interface to provide an abstraction for
managing active UID states. `ActiveUids` now implements this interface,
exposing its core methods publicly.

Update `OomAdjuster` and `OomAdjusterImpl` to utilize the new interface
type for UID state references, enabling more flexible and testable
implementations.

Bug: 425766486
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Flag: EXEMPT pure refactor

Change-Id: I40db690023e731d745dffa144fe853c5dd86f0a8